Motor Reflexes (Karius) Flashcards
What is the general purpose of a reflex?
Protective, required movement (can happen with no voluntary control of muscle)
Features of a reflex
Very specific motion and very fast
Also very precise and can look like voluntary movement. It is much faster than voluntary movement
Reflexes vs Voluntary
Reflexes do not required cortex involvement
Involuntary
Initiated by sensory input with no higher cognition
Fixed circuitry
Fast and high specificity
Myotatic reflex
Contraction of stretched muscle to protect from tearing due to stretch (basically opposite of golgi tendon)
What initiates the myotatic reflex?
Muscle spindle.
Reflex is monosynaptic and goes out to the same segment of the spinal cord
Draw the skeletal muscle including fusiform capsule, muscle spindle, intra and extrafusal fibers. Label the sensory and motor components and where the nuclear bag and chains are located.
Ok
Characteristics of the sensory component of the muscle spindle
Not contractile, sensitive to length
Contains nuclear bag and nuclear chain
Primary afferent vs secondary afferent
1a/1b fibers (large myelinated) vs group II fiber (small, myelinated)
- innervates both the bag and the chain
- innervates the chain
1a fiber makes the sensory component sensitive to…
Length of muscle, and how fast the length is changing
What happens to the sensory component when there is stretch (change in length) detected?
How about if the stretch is fast?
Frequency of AP is increased compared to unstretched.
Since 1a fibers are sensitive to speed, the AP frequency will increase more
What does Group II fiber make sensory component sensitive to?
Only length of muscle, not speed
What innervates the motor component?
What does it control?
Gamma motor neuron,
controls the intrafusal fibers, thereby controling the length of the sensory portion
What happens when the motor component stretches the sensory portion?
Sensory part becomes more sensitive to more stretch due to increase sensitivity of the 1a and II fibers to stretch (stronger stretch response)
Describe the myotatic reflex pathway
1a afferent enters the dorsal root > release EAA at synapse > synapse to alpha motor neuron > contract the stretched target muscle
1 synapse system
Explain what happens to the antagonist muscle during the myotatic reflex:
1a afferent synapses to an inhibitory neuron by releasing EAA > inhibitory interneuron will release inhibitory GABA > synapse to the alpha motor neuron of the antagonist muscle > inhibits it and it generates fewer action potentials > relaxing (stretching) the muscle
*1a afferent also sends info to the cortex informing the brain of the reflex event
What is the role of antagonist stretching in the myotatic reflex?
If antagonist is relaxed, it cannot oppose the contraction of the agonist, so the myotatic reflex can happen
How is the Golgi tendon reflex different from the myotatic reflex?
Relaxes a contracted muscle suddenly. Purpose is to protect the muscle from damage if it contracts too strongly
Characteristics of the Golgi tendon reflex?
Initiated by the golgi tendon organ
2 synapses
What are golgi tendon organs?
1b fibers that innervate the tendons
Requires contraction of the target muscle (lots of action potentials that increase with tension).
Explain the pathway of the golgi tendon reflex
1b afferent enters the spinal cord via the dorsal horn > release of EAA on inhibitory interneuron > GABA is released by inhibitory interneuron and synapses on alpha motor neuron of the contracting muscle > inhibits it and causes the muscle to relax suddenly
*GTR is a high threshold reflex. Needs lots of contraction to stimulate the interneuron
Cortical reflexes:
hopping reaction
placing reaction
Brainstem/midbrain reflexes
vestibular righting suckle yawn eye and head movements
spinal reflexes
stretch
golgi tendon
crossed extensor
What happens to alpha motor neurons after a spinal cord transection?
Spinal shock. Loss of input from the upstream neurons/cortex. Loss of all reflexes below the lesion
What is axonal sprouting and how does it occur?
5HTC receptor is expressed (self-activating)
What is the reason for hyperactive reflexes after brain injury?
Brain inhibitory region needs input from the cortex to work. Without this, it is unable to inhibit gamma motor neurons. BFC activates, but no inactivation, gamma motor neurons firing rate is altered = more active reflex
Patient presents with flexed upper limbs, extended and internally rotated lower limbs. Position seems to be dependent on head position.
What condition does this refer to?
Where is the lesion?
Decorticate posturing
Lesion is above the red nucleus
Patient presents with extended upper and lower limbs. Position seems to be independent of head position.
What condition does this refer to?
Where is the lesion?
Decerebrate posturing
Lesion is caudal to the red nucleus. Loss of input from areas rostral to pons.
Patients with spasticity present with what symptoms?
What is the usual cause?
resisting passive muscle stretch.
Hyperactive myotatic reflex (due to overactive gamma motor neurons).
Cortex damage > inhibition of brainstem inhibitory region
Patients with rigidity present with what symptoms?
What is the usual cause?
Muscle contraction in the absence of stimuli due to alpha motor neurons continuously firing
Cortex damage > inhibition of medullary influence to alpha motor neurons > overactive alpha motor neurons
